Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Em que data resultou o massacre com 21 feridos?
 ###Tabelas: CREATE TABLE top_scorers_in_uefa_club_competitions (top_scorers_in_uefa_club_competitions_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Player VARCHAR, Goals INT, Games INT, Debut in Europe INT) CREATE TABLE school_massacres (school_massacres_id AUTO_INCREMENT PRIMARY KEY, Perpetrator VARCHAR, Date VARCHAR, Year INT, Location VARCHAR, Country VARCHAR, Killed INT, Injured VARCHAR, Additional Notes VARCHAR) CREATE TABLE by_ownership_of_goods_and_services (by_ownership_of_goods_and_services_id AUTO_INCREMENT PRIMARY KEY, Region VARCHAR, Vehicle VARCHAR, Automatic washing machine VARCHAR, Refrigerator VARCHAR, Water heater VARCHAR, Fixed telephone line VARCHAR, Mobile phone VARCHAR, Cable/Sat. TV connection VARCHAR) CREATE TABLE "_"_45_nm ("_"_45_nm_id AUTO_INCREMENT PRIMARY KEY, Model number VARCHAR, sSpec number VARCHAR, Frequency VARCHAR, Turbo VARCHAR, Cores INT, L2 cache VARCHAR, L3 cache VARCHAR, I/O bus VARCHAR, Mult. VARCHAR, Memory VARCHAR, Voltage VARCHAR, Socket VARCHAR, Release date VARCHAR, Part number(s) VARCHAR, Release price ( USD ) VARCHAR) CREATE TABLE fourth_round_proper (fourth_round_proper_id AUTO_INCREMENT PRIMARY KEY, Tie no VARCHAR, Home team VARCHAR, Score VARCHAR, Away team VARCHAR, Date VARCHAR)  
SELECT Date FROM school_massacres WHERE Injured = 21